1. Field of the Invention
The present invention relates to the wavelength domain optical switch which makes it possible to use a cheap lens, makes it possible to correct aberration of the demultiplexed wavelengths produced in a plurality of waveguide type demultiplexing circuits, and is no depending to polarization of incident light.
2. Description of the Related Art
As shown in FIG. 6, a wavelength domain optical switch 600 described in the specification of U.S. Patent Application Publication No. 2006/67611 is constituted by input/output optical fibers 601 to 606, a collimator lens array 610, a Wollaston prism 615 constituted by two triangular prisms 616, 617 for making a characteristic between horizontal polarized light (y polarization) and vertical polarized light (x polarization) independent, a birefringent plate 620 for setting a phase difference between the horizontal polarized light and the vertical polarized light at zero, a ½-wavelength plate unit 625 constituted by a ½-wavelength plate part 626 and a part 627 that does not affect the polarization, a concave mirror 630, a cylindrical lens 635, a grating 642 having an edge prism 641, a prism 646 for bending light vertically, and an LCOS SLM (Liquid Crystal On Si Spatial Light Modulator) 645.
Further, as shown in FIGS. 7A and 7B, a waveguide type wavelength selective switch described in the specification of U.S. Pat. No. 7,088,882 employs a MEMS (Micro Electro Mechanical System) micromirror 701. Here, five waveguide type demultiplexers 703 are disposed on a single substrate 702, and five further substrates 702 are stacked.
There is the following problem in the related art of FIG. 6.
(1) Since a bulk type grating 642 is used, and therefore a plurality of input beams is dispersed by the single grating 642. However, the dimensions of the bulk type grating 642 are large, making it difficult to achieve a reduction in size.
(2) The collimator lens array 610 is used for the respective input/output optical fibers 601 to 606, and since the collimator lens array 610 must be aligned with the input/output optical fibers 601 to 606 extremely strictly, a large amount of time is required for assembly. Furthermore, in order to suppress aberration, the collimator lens array 610 must be formed in an aspherical shape, leading to a large increase in cost.
(3) Since a complicated optical system is used, the price and assembly cost of the respective optical component increase. Hence, it is difficult to achieve a reduction in cost.
There is the following problem in the related art of FIG. 7.
(1) The plurality of waveguide type demultiplexers 703 is disposed in planar form on the single substrate 702. When the MEMS micromirror 701 is used, a large reflection angle is permitted, and therefore this structure is possible. However, when this structure is applied to a wavelength domain optical switch such as that of the present invention, the reflection angle of the LCOS SLM is small, and therefore the performance deteriorates dramatically. Further, to increase in the number of switchable ports in the related art, the plurality of substrates 702 (five in this example) on which the respective waveguide type demultiplexers 703 are disposed in planar form are laminated in a thickness direction, and a lens array 704 on which light converges in a vertical direction is provided for each substrate 702. However, the lens arrays 704 must be aligned with the respective waveguide demultiplexers 703 extremely strictly, leading to an increase in the amount of time required for assembly. Further, in order to suppress aberration, the lens array 704 must be formed in an aspherical shape, leading to a large increase in cost. These difficulties become gradually more insurmountable as the size of the lens array 704 is reduced, and it is therefore extremely difficult to achieve a size reduction.
(2) Spectral characteristics (demultiplexed wavelengths or center wavelengths) of the waveguide type demultiplexers 703 provided on the laminated substrates 702 must be strictly aligned such that deviation therebetween is no more than 1% of a demultiplexing interval, for example no more than 0.01 nm in the case of a 1 nm demultiplexing interval, and at current levels of microprocessing precision, it is extremely difficult to achieve this control. Accordingly, yield is extremely poor.
A common problem of the related art of FIGS. 6 and 7 is that the aberration required of the lens arrays is extremely exact, and therefore the lens arrays must be formed in an aspherical shape, leading to an increase in cost. Further, the lens arrays must be aligned with the optical fibers (or the waveguide type demultiplexers) extremely strictly, making mass production extremely difficult.